Allow _ as "don't care" shorthand for the Default contract group

Since now we might want to write flags afterwards, e.g., `<Group, flag1, flag2>`, programmers who want the default group `<Default, flag1, flag2>` might expect to be able to equivalently write `<_, flag1, flag2>`, so let's make that work

Until now, programmers would just omit `<Default>` and that continues the work as the basic default, as before

Author: hsutter

source/to_cpp1.h

@@ -4536,7 +4536,10 @@ class cppfront
         auto name = std::string{"cpp2::Default"};
         if (n.group)
         {
-            name = print_to_string(*n.group);
+            auto group = print_to_string(*n.group);
+            if (group != "_") {
+                name = group;
+            }
             if (
                 name == "Default"
                 || name == "Bounds"